Polyacetal resin has been widely used in wear parts for automobiles, and electrical and electronic appliances because of its balanced mechanical properties and excellent wear and abrasion resistance, chemical resistance, heat resistance, and electrical characteristics. However, there is an ever-increasing demand to satisfy the more strict requirements for expanding end-use applications. One such requirement is to further improve the sliding characteristics of polyacetal resin (i.e., improve the anti-friction characteristics), and to maintain the same for prolonged time periods. Representative examples of wear parts that require improved sliding characteristics include guide rollers, and guide poles for use in the magnetic tape drive systems of VTR's, 8 mm videos and the like.
Polyacetal resin has been previously used as a material from which guide rollers and the like have been made. However, magnetic tape drives have more recently been designed to operate at higher tape speeds. It has therefore become increasingly more difficult for conventional polyacetal resin to satisfy various performance requirements demanded of wear parts in contact with magnetic tape running at such higher speeds. For example, wear parts in contact with higher speed magnetic tape must maintain a variety of performance characteristics over prolonged time periods, such as, sliding characteristics without an increase in the torque of the tape drive, little or no fusion of the contact surface, and/or low noise characteristics during use.
Proposals have been made to add other resins, such as fluororesins, polyolefin resins, or silicone resins so as to improve the sliding characteristics of polyacetal resins. Other proposals include incorporation of a solid lubricant, such as graphite or molybdenum disulfide, and addition of fatty acids, fatty acid esters, silicone oils, or various mineral oils into the polyacetal base resin. However, polyacetal wear parts, such a guide rollers are required to exhibit excellent dimensional accuracy in addition to adequate sliding characteristics and, if necessary, must be capable of being machined. In this respect, the incorporation of other resins, such as fluororesins or polyolefin resins, is likely to cause "burrs" and "fuzz" to be formed on the wear part's contact (sliding) surface. As a result, torque fluctuations of the tape drive and increased abrasion may occur.
The use of known oil lubricants is also not entirely satisfactory, since they tend to "bleed" onto the surface of the polyacetal resin, particularly at high temperatures. As a result, wear parts formed of an oil lubricant-containing resin usually cannot be grasped adequately by screws and/or exhibits insufficient plasticization during molding. In an extreme case, it may be impossible to mold such a resin composition. Even if the molding is successfully accomplished, the oil that "bleeds" onto the surface of the molded article renders it entirely unsuitable for use as a part in contact with magnetic tape--i.e., since the oil may contaminate the tape degrading its function.
Furthermore, although conventional techniques of improving the sliding characteristics of polyacetal resins do, in fact, initially improve the sliding characteristics of molded articles formed thereof, the initial improvements degrade over time. Thus, the use of conventional molded articles for prolonged time periods causes the sliding surface characteristics to change. As a result, increased torque in the tape drive often occurs.
As described briefly above, it is very difficult to prepare resin compositions according to known techniques to produce wear parts that can be easily fabricated and exhibit excellent short-term and long-term sliding characteristics, and excellent machinability. More particularly, what has been needed in the art is a polyacetal wear part (e.g., guide rollers or guide poles for VTR's, 8 mm video machines or the like), having balanced sliding characteristics--that is, having excellent wear and abrasion resistance with respect to metallic shafts of a high-speed tape drive--and which maintains such characteristics for prolonged periods of use. It is towards providing such a need that the present invention is directed.
According to the present invention, improved anti-frictional wear parts are provided. More specifically, wear parts according to the present invention are fabricated from a polyacetal resin composition that includes an inorganic powder having a particular particle diameter and particle diameter distribution, and a specific fatty acid ester. The polyacetal resin composition described above exhibits excellent wear part performance characteristics, particularly when the wear part is in the form of a guide roller or a guide pole (which rotates on a metallic shaft) of a magnetic tape drive for a VTR or 8 mm video machine.
The present invention more specifically relates to a wear part (e.g. tape guide rollers and/or guide poles) formed of a polyacetal resin composition which includes:
(A) a polyacetal base resin;
(B) between 1 to 20% by weight (based on the total composition weight) of an inorganic powder having an average particle diameter of 50 .mu.m or less, and a particle diameter distribution such that at least 95% of particles have a diameter of 100 .mu.m or less; and
(C) between 0.05 to 10% by weight (based on the total composition weight) of a fatty acid ester which is the ester reaction product of a fatty acid having 5 to 32 carbon atoms with a monohydric or polyhydric alcohol having 2 to 30 carbon atoms.
Wear parts of the present invention are preferably formed of a polyacetal resin composition that includes calcium carbonate or silica as the inorganic powder component (B), and stearyl stearate, pentaerythritol monostearate, pentaerythritol tetrastearate or behenyl behanate as the fatty acid ester component (C). Stearyl stearate and pentaerythritol tetrastearate are especially preferred.
Further aspects of the present invention will become more clear after consideration is given to the following detailed description of the preferred exemplary embodiments thereof.